The present invention relates to an improved guidance device for use with an earthworking machine, and more particularly with a trench digging machine.
Guidance devices for use with trench digging machines, and more particularly with drain tile laying machines, are already known.
To properly drain a field, it is necessary to lay the drain tile at a slight slope. It is therefore necessary to dig the trench in which the tile is to be laid, at a slight slope. This normally involves the use of guidance means which automatically directs the trench digging machine to dig the trench at the desired slope.
A known guidance device for automatically directing a trench digging machine, comprises a signal emitter, such as a laser gun, which is located in the field at a reference point higher than the highest point to be dug. The signal emitter emits from the reference point a signal beam, such as a laser beam at an angle matching the desired slope. A signal receiver which is located at the top of a telescopic mast mounted on the plow of the trench digging machine, receives the laser beam and actuates suitable operating means which adjust the depth of the trench digging element, or plow, of the machine. To allow for proper laying of the drain, the telescopic mast, the signal receiver and the plow on which they are mounted, are adjusted at given intervals with respect to the line defined by the signal beam, depending on the average inclination of the field to be drained, so that the drain may be laid substantially at constant depth with respect to the surface of the ground while following the desired slope.
When starting digging the trench, the plow is set at a desired depth and the mast is set so that its receiver receives the signal from the emitter in the middle of its receiving surface area. Then the plow begins to move relative to the emitter and to dig the trench along a programmed line. This programmed line is previously defined by a land surveyor to compensate the irregularities of the ground while substantially matching the desired slope at which the trench is to be dug.
To allow the plow to follow such a programmed line, the mast is adjustable in height and manually programmable so that the receiver may continue to receive the signal of the emitter whatever may be the inclination of the ground to be dug. If the surface of the ground is inclined upwardly with respect to the programmed line, the operator of the plow may manually program the mast so that it lowers at such a continuous speed that the receiver continues to receive the signal of the emitter in the middle of its receiving area while the plow continues to move a constant depth with respect to the upwardly inclined surface of the ground. If the surface of the ground is inclined downwardly with respect to the programmed line, the operator may also manually monitor the mast so that it raises at continuous speed, to hold the plow at constant depth with respect to the downwardly inclined surface of the ground. Such manual monitoring permits the plow to dig the trench at a constant depth while substantially following the desired slope.
If the plow begins to move away from its programmed line because of some irregularities in the field, the signal begins to be received by the receiver away from its receiving surface area. This signal shifting actuates the operating means of the plow which in turn move the plow until the emitted signal is again received in the middle of the receiving surface area. Thus the depth of the plow is permanently adjusted and/or corrected to obtain the proper slope.
Such a guidance system is usually satisfactory. However, problems are encountered when digging a trench either in a rocky or hard ground or in a soft ground. In striking a rocky or hard ground, the nose of the plow may be deflected and thus change its attitude without necessarily changing in depth or level.
In entering a soft ground, the whole plow may sink in the ground and thus change in depth, without necessarily changing in attitude. However, in both cases, the receiver located on the plow will move with the latter and incorrectly signal a depth change causing the trench to be incorrectly dug.
Actually, the already known guidance system cannot readily and automatically distinguish between a plow depth change and a plow attitude change.